I noticed this on trying to open a definition containing LadyBug Tools component and each component produces this exception. Then running the EditPythonScript command independently in Rhino (which I saw in other posts for this problem) also displayed this error. This only happens when running Rhino from Rhino.Inside.Revit, standalone doesn’t have this problem.
The last time I opened this definition without problems was before updating to Revit 2026.5 which updated its runtime to .NET 10. I disabled all other Revit addins but that didn’t fix it.
Rhino: 8.35.26251.13001 (Rhino 8)
Revit: 2026.5 (26.5.0.55)
CLR: 10.0.12 (10.0.12-servicing.26422.108+95017c711e6afc1085133d440e42b4bd78155701)
OS: Microsoft Windows NT 10.0.19045.0